One method for providing overcharge protection is described by Xu, Chem. Reviews, 2004, 104 pp. 4303-4417. Xu describes wide range of electrolyte additives which provide both reversible and irreversible, or shutdown overcharge protection. The reversible protection is afforded by the so-called “redox shuttle” agents in which the redox additive is transformed into its oxidized form at the surface of the overcharged positive electrode. This oxidized form diffuses back to the negative electrode where it is reduced back to its original form. This reversible couple provides a system which can limit the cell potential during overcharge. While such systems offer the advantage of some reversibility and thereby potential protection against repeated overcharge excursions within the cell, there are limitations. Such shuttle chemistry can be limited by diffusion and concentration and therefore typically operates at rates below 1C. Furthermore this shuttle chemistry can generate heat and temperature increases in the cell which lead to irreversible changes in the cell. Compositions based on substituted ferrocenes and substituted aromatics, particularly those based on methoxy-substituted aromatics have been used in these applications. These methods are also described in U.S. Pat. No. 6,045,952; hereby incorporated by reference
Xu also describes a class of “shut-down additives,” which undergo irreversible chemistry in lithium ion cells and, thereby, serve to “shut down” the charging of a cell on overcharge. Such additives trigger an irreversible, one time charging cut-off and usually achieve this through: i) gas generation, which can open a current interrupter device, or ii) through polymerization, which lead to reduction of current flow due to resistance increase in the cell. Pyrocarbonates and to a lesser extent biphenyl are effective gas generators on overcharge, and biphenyl and cyclohexylbenzene are able to undergo polymerization at the overcharged positive electrode triggering resistance increase within the cell. While such additives can be effective at moderate rates of overcharge of about 1C, their ability to prevent thermal runaway reactions at higher rates of overcharge is typically limited.
In US20050227143 A1, Amine et. al., describe the use of the fluorinated dodecaborate electrolyte salts disclosed in US20050053841 A1 and US20050064288 A1 as electrolytes which can provide redox shuttle additives that are capable of providing improved overcharge protection. The disclosure of the foregoing patent application publications is hereby incorporated by reference. Because the salts are also electrolytes and can be used in higher concentration than previous shuttle molecules, they allow overcharge protection at rates between about 1 and about 3 C. While usage of such salts in greater concentrations allows enhanced overcharge protection in comparison to standard redox shuttle additives, in some cases these salts may have limited reversibility when the system is overcharged.
Lee, H.; Lee, J. H.; Ahn, S.; Kim, H. J.; Cho, J. J.; Electrochemical and Solid State Letters, 2006, 9, (6), pp. A307-A310 has disclosed that a combination of 2 shut-down additives, biphenyl and cyclohexylbenzene has a synergistic effect by increasing the rate and thickness of resistive film formation at the positive electrode on overcharge.
There is a need in this art for an electrolyte and lithium ion battery having improved overcharge protection.